The Evolution of Compact Three Axis Accelerometers

STMicroelectronics and Bosch Sensortec are the market leaders in the compact three-axis accelerometer space. Bosch was the first to market with a 3 mm x 3 mm device, the SMB380, which was launched in early 2007. A few months later ST launched the LIS331DL. Chipworks has performed detailed reverse engineering analysis on both of these devices. Recently, we have analyzed samples of 2 mm x 2 mm devices from both Bosch and STMicroelectronics. In the following we report on some of our key findings.

The SMB380 came packaged in a 3 mm x 3 mm x 0.9 mm QFN type package. Bosch subsequently launched the BMA150, which contained the same two die, but in an LGA package. The SMB380 MEMS and ASIC die were placed side-by-side in the package, which allowed Bosch to quite easily achieve a 0.9 mm package thickness. The MEMS die area is 2.55 mm2 (see Table 1). The MEMS sensor was comprised of a separate proof mass structure for X,Y and Z axis sensing, with the total area of the sensor being 0.38 mm2, corresponding to 15% of the die area or a rather paltry 4% of the 9 mm2 package area.

Name

PKG

X

mm

PKG

Y

mm

MEMS

Die

Area mm2

Sensor

area

mm2

Sensor

Area

% Die

Sensor

Area

% Pkg

ASIC

Area

mm2

Geometry

Bosch SMB380

3.00

3.00

2.55

0.38

15

4

1.70

side-by-side

STMicroelectronics

LIS331DL

3.00

3.00

3.90

0.77

20

9

2.88

stacked

Bosch BMA220

2.00

2.00

1.37

0.22

16

6

1.33

stacked

STMicroelectronics

A2L

2.00

2.00

2.03

0.75

37

19

1.73

stacked

Hypothetical 1×1

1.00

1.00

–

0.19

–

0.19

–

–

Table 1 Summary of Package and Die Dimensions

The LIS331DL is packaged in a 3 mm x 3mm x 1 mm LGA package. Our reverse engineering found that ST had chosen to stack the MEMS and ASIC die, and hence they needed to thin both of the die to achieve a 1 mm thickness. The stacked geometry allowed the MEMS die to be 50% larger, at 3.9 mm2, as compared to the SMB380. Furthermore, the MEMS sensor is twice as big at 0.77 mm2, giving the sensor area to be about 9% of the package area.

Bosch announced a 2 mm x 2mm device, the BMA220, in early January 2010. Our analysis found that achieving the better than 50% shrink in the package area required Bosch to adopt a stacked geometry for the MEMS and ASIC die. They also reduced the MEMS die area by nearly 50% to 1.37 mm2, and the MEMS sensor area to 0.22 mm2. The end result was that the sensor area corresponded to 6% for of the package area, which is somewhat larger than for the SMB380.

A comparison of the specifications for the LIS331DL, the SMB380 and the BMA220 are given in Table 2. Since the sensor area is so much smaller for the BMA220, and it uses a single proof mass for XYZ sensing, it is not surprising that the specification BMA220 are somewhat poorer than those for the 3 mm x 3 mm devices.

LIS331DL

SMB380

BMA220

Range

±2g, ±8g

±2g, ±4g ± 8 g

±2g, ±4g, ±8g, ±16g

Sensitivity

2g: 18 mg/digit

8g: 72 mg/digit

2g: 256 LSB/g (3.9 mg/digit)

4g: 128 LSB/g (7.8 mg/digit)

8g 64 LSB/g (15.6 mg/digit)

2g: 16 LSB/g (62 mg/digit)

4g: 8 LSB/g (125 mg/digit)

8g: 4 LSB/g (250 mg/digit)

16g: 2 LSB/g (500 mg/digit)

Nonlinearity

0.5%

0.5%

2%

0g offset

±40 mg

±60 mg

±100 mg

Bits

8 bit

10 bit

6 bit

Table 2 Comparison of Published Specifications

STMicroelectronics also announced a 2 mm x 2 mm accelerometer in January 2010, at the Consumer Electronics Show in Las Vegas. They have not, as of yet, released a datasheet or ST part number. Chipworks recently found samples of these devices, labeled A2L, in a 6th generation Apple iPod Nano. Our analysis found the A2L to contain stacked MEMS and ASIC die, both with 2010 die marks. Remarkably, ST was able to keep the sensor area almost unchanged, at 0.75 mm2, from that seen in the LIS331DL. The main strategy for achieving this was the use of a hermetic gold seal for the MEMS cap, instead of the conventional frit glass seal. The result was that MEMS sensor area corresponded to 19% of the 2 mm x 2 mm package area. Both the A2L and the LIS331DL use an integrated sensor for XY sensing and a separate sensor for Z. Specifications are not available for the A2L; however, we would suspect the spec’s for the A2L to be at least as good as those for the larger LIS331DL, since the sensor area is similar.

The relative area for the package, MEMS die, MEMS sensor and ASIC die are shown graphically in Figure 1 for the four parts considered here. The figure shows the plan-view X-ray with the MEMS and ASIC die size shown in green and blue respectively. The MEMS sensor size is shown in bright green. Chipworks believes the industry is working towards 1 mm x 1 mm three-axis accelerometers. This corresponds to a further factor-of-four reduction in the package area. If it is possible to maintain the sensor area at 19% of the package area, achieved by ST for the A2L, then the sensor area would be 0.19 mm2, which is only slightly smaller than that used for the BMA220. This would be 0.43 mm on a side, assuming square geometry, as shown at the extreme right of Figure 1. The ASIC would also need to be less than 1 mm2 in area, and it would appear that Bosch is closer to achieving this than ST at the present time, since the BMA220 ASIC is only 1.37 mm2 in area. A reduction of the process by one node would give the necessary die shrink. Chipworks expects to see 1 mm x 1 mm devices from the two market leaders within a year or two. It is interesting to speculate whether the other players in the industry, such as Freescale, Analog Devices and Kionix, will try to keep up, or will the forces of consolidation and commoditization be too strong?